In many electronic devices there is a need to transmit audio data signals, or other data signals, within the electronic devices, or to some peripheral or accessory device(s) that may be attached to the electronic devices, for instance a set of headphones. In many modern electronic devices, especially devices with RF transmission capability such as a mobile telephone or tablet computing device, analogue audio signals are liable to be corrupted by electromagnetic interference (EMI) and coupling from other nearby circuitry. It is therefore desirable to transmit signals from an audio processing device, e.g. an integrated circuit audio hub/codec, to the driver of a transducer e.g. speaker, in a digital format so as to preserve the integrity and quality of the audio signal. For audio data there is also typically a requirement to send data for multiple channels, for instance left (L) and right (R) data channels for stereo audio.
Modern electronic devices such as smartphones, tablets and the like typically transmit digital audio data using three data wires plus a ground wire. The signal data for both L/R audio channels is typically sent on one wire in serial format, for example in words of 24 bits, with separate words being sent for the left (L) data channel and the right (R) data channel. The bit clock signal (BCLK) is sent on another wire and a further clock at the left/right words rate (LRCLK) sent on a further wire as illustrated in FIG. 1.
Alternatively, the audio data may be transmitted as a high-rate 1-bit stream, sometimes referred to as Pulse Density Modulation (PDM). Word-length reduction, noise shaping and/or delta-sigma techniques may be applied to reduce quantisation noise in the audio band at the expense of noise at higher frequencies so as to reduce the required bit rate. However, this still requires two wires for the transfer of digital audio data, one wire for the data and one wire for the clock. Moreover, the digital data spectrum will include components corresponding to the base band audio signals, which may couple onto other analogue lines associated with the device. Stereo data may be transmitted along one wire, typically alternating between sending left channel data and right channel data, but the separate clock line is still needed and the digital data spectrum may still present similar problems.